In cellular radio communication systems, the changeover from UMTS (Universal Mobile Telecommunication System) to LTE (Long Term Evolution) has been proceeding. In LTE, OFDM (Orthogonal Frequency Division Multiplexing) is adopted as downlink radio access technology that can achieve high efficiency in frequency utilization.
On the other hand, the 3GPP (3rd Generation Partnership Project) that has defined the LTE specification is currently conducting a study on the standardization of LTE-A (LTE Advanced) in order to achieve higher communication speeds for next generation communication systems. The study includes discussions on “carrier aggregation.” In carrier aggregation, a plurality of existing LTE carriers (hereinafter called “component carriers” for convenience) are aggregated and the thus aggregated carriers are received. FIGS. 1A and 1B illustrate examples of carrier aggregation currently under study.
FIG. 1A concerns an example of carrier aggregation in which five contiguous LTE carriers (component carriers), each with a 20-MHz bandwidth, are aggregated. Since each 20-MHz component carrier is compatible with LTE, communications with existing LTE users (LTE UEs) can be carried out by using such component carriers. On the other hand, the LTE-A user (LTE-A UE) can perform communications with a bandwidth of up to 100 MHz by using one to five CCs as desired. Aggregation of a plurality of component carriers spaced apart in frequency, such as depicted in the example of carrier aggregation of FIG. 1B, is also being studied.
Generally, in a radio communication system, a pilot signal as a known signal is inserted for transmission between the transmitter and the receiver for such purposes as timing synchronization and propagation channel condition estimation. This pilot signal is called a reference signal (hereinafter abbreviated RS). FIGS. 2A and 2B illustrate how such RSs are arranged on radio resources. In the example depicted in FIGS. 2A and 2B, the RSs are arranged in a scattered manner over two-dimensional radio resources plotted with subcarrier (frequency) as abscissa and OFDM symbol (time) as ordinate. In LTE, the arrangement of RSs is determined by the cell ID assigned, for example, to the radio base station designated as the transmitter. For example, FIG. 2A illustrates the arrangement of RSs when the remainder on dividing the cell ID of the radio base station by 6 is 0 (Cell-ID %6=0), while FIG. 2B illustrates the arrangement of RSs when the remainder on dividing the cell ID of the radio base station by 6 is 2 (Cell-ID %6=2). Further, in LTE, RSs are mapped at intervals of six subcarriers. As illustrated in FIGS. 2A and 2B, the offset from the band end (the leftmost end of the radio resources in FIGS. 1A and 1B) for starting the mapping of the RSs to the radio resources differs according to the remainder left on dividing the cell ID of the radio base station by 6. The arrangement of RSs in LTE described above is disclosed in the 3GPP's technical specification 3GPP TS 36.211.
In LTE, the mobile station as the receiver performs propagation channel estimation by using RSs which are known signals. Various methods are known for propagation channel estimation. As an example, a well known method of propagation channel estimation in time domain will be described below.
First, the mobile station performs pattern cancellation on the RSs inserted in the received OFDM signal and converts it into a signal of a prescribed waveform such as a rectangular wave. Next, an IFFT of length equal to a predetermined number (for example, Nfft) of samples is applied to a predetermined number (for example, Nc) of subcarrier signals in the pattern-canceled signal, after which processing for removing noise components is performed. Finally, an FFT is applied to convert the signal into a frequency-domain signal.
For example, when the offset is 0 (FIG. 2A), in LTE the RSs are arranged at intervals of six subcarriers, as shown in FIG. 3, and only the RSs are extracted from the received OFDM signal at the mobile station. As shown in FIG. 3, when the resource elements, i.e., the subcarriers, in the first OFDM symbol are numbered in sequence starting from the band end, pattern cancellation is performed on the 200 (i.e., the number, Nc, of) RSs mapped to the subcarriers numbered 0, 6, . . . , 1194 out of the 1200 subcarriers, and an IFFT of size Nfft=256 is applied. If the pattern-canceled signal is a rectangular wave, the IFFT output becomes a sin c function. After noise components are removed by equating to zero any portion not larger than a predetermined threshold value, the propagation channel waveform in the time domain of the IFFT output is converted by FFT into the frequency domain. A propagation channel estimate is thus obtained for each subcarrier.
In LTE-A, carrier aggregation is employed, as earlier described, but it is proposed that when aggregating 20-MHz component carriers, 19 null subcarriers (subcarriers not transmitting any signals) be inserted as a guard band between each component carrier. For example, as shown in FIG. 4, two component carriers each with a 20-MHz bandwidth are aggregated in the downlink OFDM signal to be transmitted from the base station eNB to the mobile station UE, and 19 null subcarriers are inserted between the two component carriers. The base station UE receives a signal with an aggregate bandwidth of 40 MHz and applies signal processing to the entirety of the 40-MHz signal at the baseband level.
Related art is disclosed in 3GPP TS 36.211 V9.0.0) (2009-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 9), and Contiguous Carrier Aggregation-Overall Proposal.